Hydrogen Sorption in Erbium Borohydride Composite Mixtures with LiBH4 and/or LiH

Abstract

Rare earth (RE) metal borohydrides have recently been receiving attention as possible hydrogen storage materials and solid-state Li-ion conductors. In this paper, the decomposition and reabsorption of Er(BH 4 ) 3 in composite mixtures with LiBH 4 and/or LiH were investigated. The composite of 3LiBH 4 + Er(BH 4 ) 3 + 3LiH has a theoretical hydrogen storage capacity of 9 wt %, nevertheless, only 6 wt % hydrogen are accessible due to the formation of thermally stable LiH. Hydrogen sorption measurements in a Sieverts-type apparatus revealed that during three desorption-absorption cycles of 3LiBH 4 + Er(BH 4 ) 3 + 3LiH, the composite desorbed 4.2, 3.7 and 3.5 wt % H for the first, second and third cycle, respectively, and thus showed good rehydrogenation behavior. In situ synchrotron radiation powder X-ray diffraction (SR-PXD) after ball milling of Er(BH 4 ) 3 + 6LiH resulted in the formation of LiBH 4 , revealing that metathesis reactions occurred during milling in these systems. Impedance spectroscopy of absorbed Er(BH 4 ) 3 + 6LiH showed an exceptional high hysteresis of 40-60 K for the transition between the high and low temperature phases of LiBH 4 , indicating that the high temperature phase of LiBH 4 is stabilized in the composite.

Rare Earth Borohydrides—Crystal Structures and Thermal Properties

“…Between the two studies, minor differences were observed in the number and variety of ErH 3 polymorphs formed upon rehydrogenation. While cubic-ErH 3 was the only polymorph reported in [72], a mixture of tetragonal-ErH 3 and cubic-ErH 3 polymorphs were reported after rehydrogenation (340 • C, 93 bar H 2 ) in [73].…”

“…The structure and thermal properties of halide-free RE-borohydrides (RE = Pr, Er) have also been obtained by solvent-based synthesis instead of mechanochemistry [30,73]. Both compounds showed a simple decomposition pathway into the corresponding hydrides and borides according to Equations (8) and (9) …”

“…The decomposition and rehydrogenation behavior of composite mixtures containing the halide-free Er(BH 4 ) 3 ball milled for one hour in the presence of LiBH 4 and/or LiH has been further investigated in reference [73]. Two composites were studied in detail: Er(BH 4 ) 3 -LiH (1:6) and LiBH 4 -Er(BH 4 ) 3 -LiH (3:1:3).…”

“…More recently, a selection of composites containing either La, Er or Pr have been synthesized to determine whether the composites obtained by mechanochemistry differ from those made by wet-chemical methods [30,72,73]; whether the composites which contain alkali chlorides as by-products show a different thermal behavior from those which are halide-free; and determine the role of additives such as LiH for the rehydrogenation behavior of the composites. Composite mixtures of RE-borohydrides of La, Er and LiBH 4 and LiH were obtained by mechanochemical methods [72].…”

Functional Materials Based on Metal Hydrides

“…Michael Heere and co-workers present an interesting paper concerning hydrogen release and uptake of a new type of reactive hydride composite based on a rare earth metal borohydride, erbium borohydride, Er(BH 4 ) 3 , combined with LiBH 4 and/or LiH, 3LiBH 4 −Er(BH 4 ) 3 −3LiH with a capacity of 9 wt % H 2 [20]. Sieverts-type measurements reveal a three-step desorption-absorption cycle with release of 4.2, 3.7 and 3.5 wt % H 2 for the first, second and third cycle.…”